The present invention relates to bobbins for use in sewing machines, and relates more particularly to bobbins that are furnished with a quantity of sewing thread pre-wound thereon.
In a typical lock-stitch sewing operation performed on an automatic sewing machine, a reciprocating needle carries a needle thread through a fabric from one side thereof through to the other side of the fabric, and the needle thread is looped around an under thread fed from a bobbin, such that the needle thread and the under thread form interlocking loops. The bobbin comprises a spool of thread rotatably mounted in a bobbin case located beneath the foot plate of the sewing machine over which the fabric travels.
A typical bobbin case 10 is shown in FIG. 1, along with a conventional single-flange bobbin 20. The bobbin case comprises a cup-shaped housing formed of steel. A post 12 is mounted on an end wall 14 of the bobbin case and the bobbin 20 has a hollow cylindrical core 22 that fits over the post loosely enough so that the bobbin can freely rotate on the post. The bobbin thread 24 or under thread is wound about the core. A lock-stitch sewing machine includes mechanisms for guiding the bobbin thread out from the bobbin case and looping it in interlocking relation with the needle thread. During continuous lock stitching, the bobbin 20 rotates in the bobbin case 10 as a result of the bobbin thread being drawn out from the bobbin case by the various mechanisms.
When the sewing machine operator stops the machine, the various mechanisms of the machine tend to stop abruptly so that the tension on the bobbin thread suddenly ceases, but there is a tendency for the bobbin to continue to rotate in the bobbin case until frictional forces between the bobbin and the case bring the bobbin to a halt. This phenomenon is generally known as overspin. When the bobbin overspins, slack is created in the bobbin thread, and if the overspin is great enough, the slack thread can become looped around components in the bobbin case in an undesirable fashion, resulting in breakage of the thread when sewing is recommenced. Thus, it is desirable to minimize overspin as much as possible.
The draw-off tension, or amount of force required to draw the bobbin thread out from the bobbin case, is also an important parameter affecting the quality of stitching performed on lock stitch sewing machines. Bobbin assemblies typically include a tension adjustment spring for adjusting this draw-off tension to a desirable level for the particular type of bobbin thread being used and the sewing operation being performed. A typical amount of draw-off tension is about 30 to 35 grams. Although the tension adjustment spring can affect the average level of the draw-off tension, the draw-off tension can also be affected by interaction between the bobbin and the bobbin case. For example, in the case of bobbins of the type generally referred to as sidewall bobbins, having a circular sidewall or flange 26 attached to one or to each end of the core 22 such as in the single-flange bobbin 20 shown in FIG. 1, the outer periphery of the flange(s) 26 can rub against the inner surface of the bobbin case.
Such interference between the flange(s) 26 and the bobbin case is a potential problem particularly in sewing machines that employ automated bobbin changing devices, which automatically remove the bobbin case from the sewing machine when the bobbin thread is depleted and replace the bobbin case with another bobbin case containing a fully wound bobbin. During the automatic changing operation, the bobbin case can be subject to forces and accelerations that can tend to dislodge the bobbin from the case. To retain the bobbin in the case, conventional bobbin cases include a lever arm 16 pivotally mounted on the end wall of the bobbin case and connected with a movable finger 18 that can project through an opening in the side wall of the bobbin case and engage the flange of the bobbin contained therein, as shown in solid lines in FIG. 2. The automatic bobbin changing apparatus A, schematically depicted in FIG. 2, is operable to actuate the lever arm 16 to cause the movable finger 18 to move into position engaging the flange 26 of the bobbin during the bobbin changing procedure. Once the bobbin case is installed in the sewing machine, the lever arm 16 is released so the finger 18 disengages the flange to allow the bobbin to rotate freely, as shown in phantom lines in FIG. 2.
In order for the finger to be able to properly engage the flange 26, the outer diameter of the flange must be only very slightly smaller than the inner diameter of the bobbin case. For instance, in prior single-flange bobbins made by the assignee of the present application, the flange has a nominal outer diameter of 0.865 inch, whereas the bobbin cases in which the bobbins are used have a nominal inner diameter of about 0.875 inch. Accordingly, if the flange 26 is mounted slightly eccentrically relative to the core 22 of the bobbin as a result of normal manufacturing tolerances during manufacture of the bobbin, the flange can rub against the inner surface of the bobbin case. This can cause the draw-off tension of the bobbin to be higher than optimal, and to vary to an undesirable extent.
Ideally, the draw-off tension should be constant and should be in an optimal range for the particular stitching operation being performed. Eliminating the flange(s) of the bobbin to create a so-called sideless bobbin would circumvent the problem of interference between the flange(s) and the bobbin case, but then another way of retaining the bobbin in the bobbin case during an automatic changing operation would have to be provided. Published International PCT Application WO 00/36201 describes a sideless bobbin that purports to solve this problem. The sideless bobbin comprises a hollow cylindrical core that is magnetized so that it is magnetically attracted to the steel post and end wall of the bobbin case, thus retaining the bobbin in the bobbin case during an automatic changing procedure. The sideless bobbin of WO 00/36201 is also said to reduce overspin of the bobbin and to improve the uniformity of draw-off tension as a result of the magnetic attraction force between the core and the bobbin case post and end wall.
It is believed the sideless bobbin in accordance with WO 00/36201 would have a number of drawbacks in practice. Because there must be a radial clearance between the core and the bobbin case post, the contact area between the inner surface of the core and the post must be quite small. More importantly, the contact between the core and the post can vary significantly and can even momentarily cease altogether as a result of wobbling or lateral movement of the bobbin on the post as the bobbin rotates, in which case the magnetic drag force between the bobbin and the post could momentarily drop to zero or a very low value, then jump up to a higher value when contact between the core and the post resumes. It is expected that this phenomenon would result in non-uniform draw-off tension.
A further drawback of the sideless bobbin of WO 00/36201 is that the magnetized core is magnetically attracted not only to the bobbin case, but also to the bobbin basket in which the bobbin case is installed in a sewing machine (for example, see the bobbin basket B schematically depicted in FIG. 2). Thus, as stated in WO 00/36201, it is important to assure that the magnetic attraction force between the magnetized core and the bobbin case is greater than that between the magnetized core and the bobbin basket, so that the bobbin will be extracted along with the bobbin case during a bobbin changing procedure. Accordingly, use of the sideless bobbin necessitates special accommodations to assure that this is the case. In the case where the magnetized core has uniform magnetic strength over its entire length (which is the simplest form of the core to manufacture), the patent application suggests that a gap must be created between the core and the base of the bobbin basket. This gap can be created by placing a non-ferrous washer in the bobbin basket, or by coating the bobbin basket with a non-ferrous material. It would be desirable, however, not to have to make such alterations in the conventional bobbin basket arrangement.
Thus, it is apparent based on the foregoing and the prior art that a long-standing problem has existed in sewing machine bobbin arrangements with respect to attaining a desired uniform draw-off tension and reducing overspin with pre-wound bobbins that are capable of being used in systems employing automated bobbin changing mechanisms. The problem existed as of the date of the invention described in the publication WO 00/36201, and continues to exist, but the purported solution proposed in that publication is not entirely satisfactory as noted above.
The present invention addresses the above needs and achieves other advantages, by providing a pre-wound bobbin of the sidewall type having a single flange or a pair of flanges, in which one flange of the bobbin comprises a magnetized material such that the flange magnetically adheres to the end wall of the bobbin case. The flange is sized to provide a contact area with the end wall that, in relation to the magnetic strength of the magnetized material, achieves a magnetic attraction force exceeding the weight of the bobbin so that the bobbin is held in the bobbin case against the force of gravity when the case is oriented with its open side facing downward, such as may occur during an automatic bobbin change.
Preferably, the magnetic attraction force is such that the draw-off tension due solely to the magnetized flange (i.e., not augmented by any tension adjustment spring or tack on the thread or the like) is less than about 30 grams, and more preferably less than about 20 grams. This enables the user to use a tension adjustment spring or the like to adjust the draw-off tension to be within a desirable range such as about 30-35 grams.
In accordance with a preferred embodiment of the invention, the flange is of such size in terms of surface area that contacts the end wall of the bobbin case, and the magnetized material is of such magnetic strength, that the magnetic attraction force between the flange and the end wall is about 10 to 20 times the weight of the pre-wound bobbin. In this way, the magnetic attraction force is sufficient to retain the bobbin in the bobbin case in opposition to inertial forces that may be imposed on the bobbin as a result of acceleration of the bobbin case during an automatic bobbin change.
The flange preferably is relatively flexible, which enables the flange to flex and conform to the end wall of the bobbin case so that it lies flat against the end wall. This means that surface imperfections, inaccuracies, or irregularities in the surface of the bobbin case end wall become less important in terms of affecting the draw-off tension. Preferably, the flange comprises a flexible sheet magnet formed of a flexible plastic or rubber type material impregnated with magnetized particles.
The magnetized material of the flange preferably adheres to a planar, bare steel surface with a magnetic attraction force of about 0.2 to 0.4 pounds (90 to 180 grams) per square inch of the material. The flange formed from this material preferably has a surface area of about 0.2 to 0.5 square inch. The magnetic attraction force preferably is about 25 to 60 grams for pre-wound bobbins weighing approximately 2.5 to 3 grams.
An advantage of the present invention is that the flange can be made significantly smaller than the inner diameter of the bobbin case; preferably, the flange outer diameter is equal to or less than the initial diameter of the roll of thread prior to usage of any of the thread. As a result, eccentric mounting of the flange on the core becomes less of a concern because the diameter of the flange can be made small enough in relation to the bobbin case to accommodate the largest expected eccentricity of the flange that may occur in manufacturing of the bobbin. Furthermore, if desired, the bobbins of the present invention can be used with bobbin cases that do not have fingers for engaging flanges, thus enabling the bobbin cases to be simplified.
In preferred embodiments of the invention, the core of the bobbin comprises a non-magnetic and non-magnetized material. Thus, the only magnetized component of the bobbin is the magnetized flange, which is remote from the bobbin basket in which the bobbin case and bobbin are installed in a sewing machine. Therefore, the magnetic attraction force between the bobbin and the bobbin basket is negligible, thereby increasing the likelihood that the bobbin case and bobbin will be extracted as a unit from the bobbin basket. Yet another advantage of the invention is that no additional components or coatings need to be provided in the bobbin basket for providing magnetic isolation between the bobbin and the bobbin basket. This is in contrast to the sideless bobbin of WO 00/36201, which requires a non-ferrous washer in the bobbin basket or a non-ferrous coating on the bobbin basket, at least with some embodiments of the sideless bobbin.
Bobbins in accordance with the present invention can be made as single-flange or double-flange bobbins. A single-flange bobbin has a flange mounted to one end of the core, but the opposite end of the core has no flange. A double-flange bobbin has a flange at each end of the core. Double-flange bobbins in accordance with the present invention preferably have a second flange that is non-magnetized and non-magnetic; advantageously, the second flange can be made of a flexible material such as paper, plastic, or the like. Making the bobbin in this way is advantageous not only because the second flange can be made less expensively than the magnetized flange, but also because the proper orientation of the bobbin (i.e., magnetized flange toward the end wall of the bobbin case) can be readily determined from a casual visual inspection of the bobbin, since the magnetized flange has a dramatically different appearance from the non-magnetized flange. This is in contrast to a sideless bobbin such as disclosed in the aforementioned WO 00/36201, whose proper orientation may be determinable only by carefully observing the direction (i.e., clockwise or counterclockwise) in which the thread is wound. Of course, the single-flange bobbins of the invention are also easy to properly orient in the bobbin cases.
The bobbins of the present invention are also advantageous because the contact area between the magnetized flange and the end wall of the bobbin case tends to be constant and thus the magnetic drag force on the bobbin tends to be constant, which leads to better uniformity in draw-off tension. Moreover, the contact area can be easily tailored to provide a desired amount of magnetic attraction force by changing the size of the flange. These advantages are in contrast to sideless bobbins such as disclosed in WO 00/36201, wherein the contact area between the magnetized core and the bobbin case post can vary significantly, particularly when the bobbin wobbles as sideless bobbins have a tendency to do, leading to varying magnetic drag and hence varying draw-off tension. Furthermore, the contact area between the sideless bobbin core and a given bobbin case cannot be altered to any significant extent and thus cannot be used as a design parameter for purposefully affecting magnetic drag, whereas the present invention allows the contact area to be varied to a substantial extent.
It is also believed that the bobbin of the present invention will have a reduced amount of overspin relative to the sideless bobbin of WO 00/36201, because the amount of contact area between the magnetized flange and the bobbin case end wall can be substantially greater than the contact area between the sideless bobbin core and the bobbin case post and end wall, thereby providing a greater amount of magnetic braking. Additionally, flanged bobbins in accordance with the invention are expected to have a reduced tendency to wobble in the bobbin case relative to sideless bobbins, by virtue of the substantial contact between the flange and the bobbin case end wall.